I used the custom sidebars plugin and WPML for a couple of websites now and always started with a research if there was a work flow for this. There isn’t or I didn’t find it. But this is my workaround:

For pages:
I create a sidebar for each language and assign the right sidebar to a page on the edit screen. No synchronization between the sidebars, but working for each language.

For templates:
If I dynamically have to choose s sidebar in a template I create the sidebars with a naming convention like “SIDEBARNAME LANG” (e.g. “about en”. When I load the sidebar in a template file I can use: